The application of a single stage (back mix) gas phase fluid bed reactor for the production of widely used polymers, is well established as a leading technology in the plastics industry.
In a typical single stage fluid bed polymerization reactor, the fluidized bed is finely divided polymer formed from the monomer gas, which is the fluidizing gas of the reactor. Both the monomer gas and the finely divided catalyst are continuously fed to the reactor, which is maintained at controlled conditions of pressure and temperature, and the polymer continuously formed discharges from the reactor at the rate the polymer is formed.
The polymer yield on the catalyst fed is a function of the residence time of the catalyst particles in the reactor. Since the typical single stage fluid bed polymerization reactor is a continuous back-mix reactor, the residence time distribution of the catalyst particles follows an experimental decay relationship. In other words, it is extremely broad.
In a typical operation, the unreacted monomer fluidizing gas discharged from the reactor is cooled, its composition reconstituted with fresh monomer fluidizing gas to maintain a constant steady-state, compressed, and returned to the fluid bed polymerization reactor as the fluidizing gas.
In the production of more complex polymers and co-polymers, it is known practice in the plastics industry to use more than one, typically two or three, back-mixed fluid bed reactors in series to permit changing monomer gas composition and polymerization conditions of temperature and pressure at different points in the polymerization reaction cycle to achieve desired polymers.
Typically, each back-mixed fluid bed reactor is separated from its adjacent units through feed and discharge locking devices. Each reactor is served by its own independent gas recycle and recompression system so that each reactor can be run on independently different compositions and/or combinations of monomer fluidizing gas. Since this approach is based on the polymer exiting one system and feeding to the next system in series, it has been found necessary to provide for a significant pressure drop from system to system (i.e. reactor to reactor), typically between 50 to 100 psig, to facilitate the transfer of the polymer powder.
In addition, since each back-mixed reactor system is a separate entity, the capital cost of a system of this type is high. Typically, the number of back-mixed reactors in series in a commercial installation has been limited to two or three systems by economic considerations despite the fact that there is increasing indication that a larger number of units in series to control residence time distribution and/or provide for flexible polymerization conditions would be advantageous.
It has been recognized in many polymerization systems, for the production of the more complex co-polymers, that staging monomer changes in the polymerization reaction results in superior properties such as tear strength and puncture resistance in films, as well as, improved impact strength combined with flexural strength in plastic injection moldings. There are a number of polymer products that can only be made by a multi-staged polymerization process.
This situation is further demonstrated by the fact that there are examples of multistage polymerization reactor systems being used to advantage in processes other than the gas phase fluid bed processing approach.
A multi-stage polymerization system using a liquid diluent to suspend the polymer, as opposed to utilizing gas phase fluidization and passing through a plurality of agitated reactors is described in U.S. Pat. No. 3,454,675. This liquid phase processing system, when applied to such co-polymers as propylene-ethylene, has the obvious disadvantage of dissolving the portion of the co-polymer that is soluble in the liquid diluent, thereby reducing yield of polymer when the liquid is removed.
Another reaction system for co-polymerization that has some commercial application is a horizontal stirred reactor which depends on mechanical agitators to transport the polymer through the reactor to the discharge port, while the reaction is conducted in the vapor or gas phase. Some staging of the polymerization is claimed by this system, but it is limited to a single gas phase composition. One such system is described in U.S. Pat. No. 4,710,538. While benefit is obtained by the staging of the polymer flow through the horizontal reactor, the process requires the expending of excess energy in order to mechanically agitate the contents of the reactor. The process is also not very adaptable since a single monomer gas is all that can be provided to the reactor.